Generally, there is a need for various ion guiding apparatuses for transporting ions between regions having different air pressures in mass spectrometry systems, Particularly, in a mass spectrometry system having an atmospheric pressure or near-atmospheric pressure interface, there is generally a background air pressure difference of more than 7 orders of magnitude between the ion interface and a mass analyzer, which brings difficulties to effective transport of ions between the regions having different air pressures. In the traditional mass spectrometry system design, a commonly adopted scheme is to use multi-stage differential cavities, wherein differential cavities at each stage are separated by flow limiting orifices, and the air pressure is substantially evenly distributed in the cavities at each stage, at which point conducting focusing lenses or multistage radio frequency ion guide will generally be provided in these cavities to confine the emission of ions when passing through these differential cavities, such that the ion transport air pressures can satisfy the working requirements of these ion conducting structures in combination with corresponding differential pumps, thereby effectively achieving the transport of ions to the regions of the mass analyzer and ensuring the sensitivity requirement of the mass spectrographic analysis of samples.
However, since the overall sensitivity of the mass spectrometry system needs to be ensured, it is not possible for the flow conductance of the flow limiting orifice to be too small. Generally, since ion beams are in high pressure regions greater than 100 Pa, i.e., diffusion is relatively severe in the regions pumped by rough vacuum pumps, the ion beams are typically difficult to be compressed to have diameters less than 1 mm. To avoid the loss of ion transport efficiency, the air pressure difference of ions from a rough vacuum section to a high vacuum section generally does not exceed 2 orders of magnitude. Even so, a molecular pump or diffusion pump having a pumping rate of more than 100 L/s must be arranged in this transitional vacuum region. In addition, a high vacuum pump having a similar pumping rate is also required in the region of the mass analyzer, such that the cost of the mass spectrometry system having the near-atmospheric pressure interfaces is greatly limited by the high vacuum pump with high price and large volume, and the spectrographic analysis apparatus is difficult to be made small and exquisite.
Another difficulty with ion transport under the transitional vacuum air pressure is too high ion cooling speed. That is, when ions are transported at a background air pressure greater than 10 Pa, their original kinetic energies or electric field accelerating kinetic energies obtained in a transport system will be taken away quickly due to a large number of collisions between surrounding gas molecules having heat movement speeds. Therefore, the dwell time of the ions in these ion guiding apparatuses is generally very long. Particularly for a continuously working pulse ion source-scanning mass spectrometry system or a quasi-continuously working large-mass-range high-speed time-of-flight mass spectrometry system, long dwell time means that a large proportion of the ions cannot reach a detector under their correct ion mass-to-charge ratio channel conditions, thereby affecting the sensitivity of the mass spectrometer. Meanwhile, the ions reaching the detector at inappropriate time may also cause adverse effects such as cross contamination, space charge effect, etc., further deteriorate the performance of mass spectrometers.
Generally, in order to solve these problems, during the focusing and confining of ions, other means need to be adopted to confine the divergence of the ions in the space and to reduce the flow conductance of the background gas between the differential cavities; meanwhile, the transport speed of the ions in the axial direction needs to be increased. Such problems have been recognized in some studies in the prior art. For example, in U.S. Pat. No. 7,982,183, it has been considered to replace the traditional flow limiting orifices with capillary round tube structures in order to form better flow limiting structures between stages. However, this scheme does not involve the prevention of the transport of ions in these capillary tubes from diffusion loss. Therefore, a higher ion transport rate cannot be achieved in such practical application. In addition, some other structures, such as radio frequency focused ion guides involved in U.S. Pat. Nos. 8,148,679 and 8,642,949 are used to confine the ions. However, these ion guides all have open-ended lateral openings, and cannot cause the movement of gas flows in the guiding apparatuses to be confined significantly. The dwell loss during ion transport will generally be solved by axially applying a DC voltage or a DC traveling wave. However, the introduction of the axial DC voltage requires introduction of a field adjusting electrode or a design in which focusing electrodes are separated to form sections, which not only increases superfluous electronic design but also increases the system power consumption, and lowers ion guiding stability.
Agilent has proposed in U.S. Pat. No. 6,646,258 a method for confining ions by a quadrupole confining field formed through applying radio frequency to a concave focusing electrode attached in a pipe. Similarly, in the International Patent Application WO2014001827, Fasmatech has also proposed a similar flow conducting duct internally provided with a jet stream by a plurality of superposed annular electrodes, wherein effective radio frequency confinement may also be formed because a reverse radio frequency voltage is applied to the adjacent annular electrodes. However, such designs, when the gas flow conductance is further confined by reducing the pipe diameter, may all have some difficulties in structural design. For example, a pseudo potential of the concave electrode in its concave direction is apt to become a negative value, such that ions diffusing nearby the concave surface suffer a loss. In addition, an inner-wall electrode having an extremely small pipe diameter (for example, less than 2 mm) is very difficult in processing, and when a pipe channel is formed by using a superposition method, the inner wall of the pipe cannot be made smooth, thus the ions around the wall surface are easily lost due to turbulence.